The conversion of liquid to vapor is a fundamental step in many processes. For a variety of reasons, such as time and energy limitations, it may be advantageous to make this phase change more efficient. For example, one method of achieving more efficient phase change is though the use of thin film evaporation.
Rising film evaporation is one form of thin film boiling. When water boils at a high enough rate in a vertical tube, the central area of the tube fills with steam. As shown in FIG. 8, under the right conditions, the upward movement of the steam will rapidly draw a thin film of water up the inside of the tube forming a natural thin film on the inner wall of the tube. As the feed mixture moves up the inside of the tube, more vapor is formed resulting in a higher central core velocity that forces the remaining liquid to the tube wall. Higher vapor velocities, in turn, result in thinner and more rapidly moving liquid film. But very accurate feed water level control (e.g., .+−.0.25″) is needed to achieve this mode of operation. Such accurate level control in a portable system may be difficult to achieve because the system would have to be leveled to within 1.degree. of tilt.
Thin film evaporation is typically achieved using apparatus that includes devices with very small openings or very small spraying arrangements. This apparatus can easily clog, particularly when the source liquid contains contaminants. The apparatus may also be sensitive to movement and positioning of the apparatus. What is needed is an invention that allows for an increase in the phase change efficiency similar to the efficiencies obtainable from thin film evaporation, without the limitations and sensitivities typically experienced with thin film evaporation.
For example, vapor compression distillation has proved useful for purifying liquids, e.g., turning salt water into potable water. Such devices frequently employ an evaporator chamber comprising a set of vertically oriented tubes, which tubes are heated on their exteriors. The heated tubes create vapor from a liquid that is input to the tubes through openings in the bottom of the tubes. The vapor that emerges from each tube is compressed and heat from the vapor is then transferred to the liquid in the tubes by passing the compressed vapor over the outside of the tubes. The vapor condenses as it transfers its heat and the resultant distillate is drawn off. A vapor compression distillation device is disclosed in The Naval Sea Systems Command (Sea-03Z43), Naval Ships' Technical Manual, Chapter 531, Desalination Volume 2, Vapor Compression Distilling Plants, # S9086-SC-STM-020/CH-531V2R2, 1 Sep. 1999, which is incorporated herein by reference in its entirety. The efficiency of a rising film evaporator can be characterized by the ratio of distillate output per unit time to the power input to the evaporator per unit time.
In this specification and in any appended claims, unless context requires otherwise, the term “phase change chamber” will mean any structure with at least one inlet end for introducing liquid and at least one outlet end for allowing vapor to exit. The chamber is intended to be heated externally and to allow a liquid-to-vapor phase change to occur within. Such chambers include, without limitation, evaporator tubes, that may be cylindrical, and the parallel core layers described above. Other geometries as are known for such chambers to those skilled in the art are intended to be within the scope of the invention as described in the claims.